The invention relates to a method for calibrating a rotary printing press, wherein a bearing structure for a printing cylinder is adjusted relative to another component of the printing press, and positions of the bearing structure are measured.
In a rotary printing press, e.g. a flexographic printing press, the position of the printing cylinder must be adjusted with high precision relative to other machine components, e.g. a central impression cylinder (CI), an anilox roller, the lateral frame of the machine (for adjusting the side register), and the like. In a typical flexographic printing press, a number of colour decks are arranged at the periphery of a CI, and each colour deck comprises a bearing structure for the printing cylinder and another bearing structure for the anilox roller. Each bearing structure comprises two bearing blocks that support the opposite ends of the printing cylinder and the anilox roller, respectively, and are movable relative to the machine frame in a predetermined direction (e.g. horizontal) so as to bring the peripheral surface of the printing cylinder into engagement with a print substrate (web) on the CI and to bring the peripheral surface of the anilox roller into engagement with the printing cylinder. The movements of the bearing blocks are controlled independently of one another by means of servo-motors which also permit to precisely monitor the positions of the bearing blocks. The exact positions which the bearing blocks have to assume during a print process depend among others upon the thickness of a printing sleeve and/or printing plates that are mounted on the printing cylinder.
When the printing press has to be prepared for a new print job, the printing cylinders have to be exchanged. In a known printing press, a hollow-cylindrical adapter which carries the printing plates of a printing sleeve is removably mounted, e.g. hydraulically clamped, on a mandrel that remains in the machine. In order to exchange the adapter, the bearing at one end of the mandrel is removed, so that the adapter can be withdrawn axially from the mandrel. Then, the new adapter, with the printing sleeve or plates carried thereon, is thrust onto the mandrel and is clamped thereon. Then, the bearing that had previously been removed is restored again.
In a start-up phase of the print process, the contact pressure between the printing cylinder and the CI and between the anilox roller and the printing cylinder has to be adjusted with high precision. Conventionally, this is done by first moving the printing cylinder and the anilox roller into predetermined start positions by appropriately controlling the servo-motors for the bearing blocks. Then, the print process is started, and the printing result is monitored and a fine adjustment is performed for optimising the contact pressures. This so-called setting procedure takes a certain amount of time, and, since the quality of the printed images produced during this time will not be satisfactory, a considerable amount of waste is produced.
In the European patent application EP 06 022 135.5, an automated setting procedure has been proposed which aims at reducing or eliminating this waste. According to this proposal, the geometry of the printing cylinder is precisely measured beforehand, for example while the printing cylinder is supported in a mounter which is used for mounting the printing plates thereon. The geometry data of the printing cylinder are then transmitted to a control unit of the printing press and are used for adjusting the bearing blocks precisely to the optimal positions which assure a good print quality from the outset.
In any case, whether the setting procedure is performed automatically or manually by try-and-error, a calibration process is necessary for assuring that the positions of the bearing blocks that are measured and monitored by means of the servo-motors or by means of separate measuring devices reflect the actual physical positions of the axes of the printing cylinder and the anilox roller with high precision. This calibration procedure implies that exact reference positions are determined for each degree of freedom of the bearing structures. When the printing press has once been calibrated and the printing sleeve is exchanged, the reference positions can be used for determining the start positions or set positions of the printing cylinder and the anilox roller that correspond to the thickness of the new printing sleeve.
In a conventional calibration process, a gauge representing the thickness of the printing sleeve or plates is manually inserted between the CI and the printing cylinder, and the printing cylinder is moved against the CI until the gauge is clamped with a suitable force. Then, the actual position of the printing cylinder is measured and stored as the reference position. The same procedure is then repeated for the anilox roller.
This procedure requires a considerable amount of skill and experience and nevertheless has only a low reproducibility, because it is left to the personnel to judge whether the gauge is clamped with suitable pressure.